1. Field of the Invention
The present invention relates generally to systems for injecting chemicals into pipelines and, more specifically, to an improved system and method for adding odorant to natural gas or liquefied petroleum gas flowing in a pipeline.
2. Description of the Prior Art
There are many instances in which it is desirable to inject chemical of various types into fluids (gas and liquids) flowing in pipelines. One such example is in the area of natural gas pipelines. In addition to such substances as corrosion inhibitors and alcohol to inhibit freezing, odorants are commonly injected into natural gas pipelines. Natural gas is odorless. Odorant is injected into natural gas in order to provide a warning smell for consumers. Commonly used odorants include tertiary butyl mercaptan (TBM). Such odorants are typically injected in relatively small volumes normally ranging from about 0.5 to 1.0 lbs/mmscf.
The odorants are typically provided in liquid form and are typically added to the gas at a location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. In such circumstances, the gas pressure may be stepped down through a regulator from, for example, 600 psi or more, to a lower pressure in the range of 100 psi or less. The odorants can also be added to the main transmission pipeline in some situations.
As will be apparent from the above discussion, the odorants which are added to natural gas are extremely concentrated. Odorants such as TBM and other blends are mildly corrosive and are also very noxious. If the job of injecting odorant is not performed accurately, lives are sometimes endangered. It would be possible for a homeowner to have a gas leak with a leak not being realized until an explosion had resulted if the proper amount of odorant was not present. Also, if a leak of odorant occurs at an injection site, people in the surrounding area will assume that a gas leak has occurred with areas being evacuated and commerce being interrupted. Contrarily, if such mistakes become common, people in the surrounding area will become desensitized to the smell of a potential gas leak and will fail to report legitimate leaks.
Two techniques are commonly used for providing odorization to natural gas in a main distribution pipeline. The first technique involves the injection of liquid odorant directly into the pipeline through the use of a high pressure injection pump. The odorant is pumped from a liquid storage tank into a small pipe which empties directly into the main gas pipeline. Because odorant is extremely volatile, drops injected to the pipeline immediately disperse and spread throughout the gas in the pipeline. In this way, within a few seconds, the drops of liquid odorant are dispersed in gaseous form. U.S. Pat. No. 6,208,913, issued Mar. 27, 2001, to Marshall and Zeck, and U.S. Pat. No. 5,490,766, issued Feb. 13, 1996, to Zeck, both show state of the art fluid pumps for injecting odorant.
There are several disadvantages with this prior art technique. As mentioned above, the odorant liquid is extremely noxious. The injection pump must therefor be designed so that no odorant can leak. This requires a pump design which is relatively expensive and complex in order to meet the required operating conditions. In even such sophisticated systems, there is an unpleasant odor present when working on the pump which can make people think that there is a natural gas leak.
Another technique for odorizing a natural gas pipeline involves bypassing a small amount of natural gas at a slightly higher pressure than the pressure of the main distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of odorant while it is in the tank. This heavily odorized bypass gas is then placed back into the main pipeline. The odorant, now volatilized, is placed back into the main pipeline and diffuses throughout the pipeline in much the same manner as described with respect to the liquid injection system. U.S. Pat. No. 6,142,162, issued Nov. 7, 2000, to Arnold, shows such a method for odorizing natural gas in a pipeline utilizing bypass piping in conjunction with a liquid odorant storage tank.
There are also a number of disadvantages associated with the bypass system for odorizing pipelines. One disadvantage of the bypass system is the fact that the bypass gas picks up large and inconsistent amounts of odorant from the liquid in the tank and becomes completely saturated with odorant gas. As a result it is necessary to carefully monitor the small amounts of bypass gas which are used. Also, natural gas streams typically have contaminates such as compressor oils or condensates which can fall out into the odorant vessel in bypass systems. These contaminates create a layer that reduces the contact area between the liquid and the bypass stream. This necessarily degrades the absorption rate of the stream.
In U.S. Pat. No. 7,056,360, entitled “Optical Odorization System”, issued Jun. 6, 2006, to Zeck, there is shown an improved system for odorizing natural gas flowing through a pipeline by injecting odorant into the pipeline at a controlled rate. The system includes an odorant storage tank containing an odorant to be injected. A pressurized source of inert gas, such as nitrogen, communicates with the odorant storage tank for maintaining the tank at a desired positive pressure above the pressure of the natural gas pipeline. An injection conduit communicates the odorant storage tank with the pipeline. A photooptic metering means, located within the injection conduit, meters odorant to be injected into the pipeline.
The odorization system described in issued U.S. Pat. No. 7,056,360, was extremely simple as compared to existing positive displacement pump systems. The system offered improved ease of understanding, operation and maintenance. As compared to the prior art systems, the system described therein offered more accurate control, verification (drop counting) and communication (alarms, status, etc) than the prior art systems. The system offered generally smell free operation and maintenance. However, despite these many advantages, the previously described system also required that the odorant storage tank be pressurized to a pressure above the pipeline pressure of the pipeline being odorized.
U.S. Pat. No. 7,389,786, entitled “Ultrasonic and Sonic Odorization Systems,” issued Jun. 24, 2008, to Zeck, represented a further advance in the art and offered certain advantages over the previously described optic system in some situations in that an ultrasonic measuring unit allows the odorant to be metered on a drop wise basis with drops of chemical being counted as they pass through the flow vale into the injection conduit and into the natural gas pipeline. The ultrasonic measuring unit also allows steady state flow conditions to be measured accurately. Such a sonic measuring unit can advantageously be utilized in a wide range of flow situations. However, the system continued to require that the odorant tank be maintained above the pressure of the gas pipeline being odorized, however.
There continues to be a need for improvements in odorization systems of the above described types.